Relation of Crank and Eccentric. 637 and x as INSIDE OR EXHAUST LAP, forming an additional width to the valve face, in line with valve spindle, on the steam or exhaust edges of the valve respectively, for the purpose of giving early cut-off to steam or exhaust. By adding steam lap the width of opening is decreased, which is, however, compensated by giving increased travel to the valve. Inside lap is rarely necessary, the alterations in valve position caused by introducing stearn lap usually giving a sufficiently early cut-off to exhaust (compression point). Various interesting points are raised by altering the proportions of the slide valve, which will be fully investigated later. (See p. 772.) Relation of Crank and Eccentric.—The commonest valve gear is the eccentric and rod. The eccentric is merely a convenient form of crank whose pin is so enlarged as to envelop the shaft: it follows that the eccentricity or length of eccentric crank must be measured from centre of eccentric sheave to centre of shaft. This amount we shall sometimes call the throw. While, then, the piston moves the crank, the latter in turn moves the eccentric, and so automatically, by the slide valve, adjusts the supply of stearn. ( Without lap.} A normal valve must of necessity be at half stroke when the piston is at the end of its stroke—that is, when ,the crank is at a dead centre^ for then the valve should be just opening to steam. The eccentric crank must therefore be placed at 90° to the engine crank. Further, the direction of rotation will be determined by the position, right or left of it, of the eccentric. The eccentric will always lead the crank or travel before it; for, if we endeavour to turn oppositely, "we shall only close the steam port at the very time itf-should be opening, and so block the supply. Therefore, in a normal valve, the eccentric must lead the crank by 90°. ( With lap.) Let us next consider a valve having lap. Re- ferring, again, to Fig, 634, the thin outline shews a valve with lap, placed at mid-stroke. It then covers the steam port plus the lap. The crank being on dead-centre F, it follows that, in order to admit steam by port R, valve must be moved bodily to the right, and the eccentric lead the crank by 90° + /#/, as at HX. A little consideration will shew that strictly similar conditions obtain with the crank on the dead-centre z.